Glutathione S-transferase-catalyzed conjugation of bioactivated aflatoxin B sub(1) in rabbit lung and liver

• Published in: Toxicology and applied pharmacology 1996-10, Vol.140 (2), p.499-507
• Main Authors: Stewart, R K, Serabjit-Singh, C J, Massey, TE
• Format: Article
• Language: English
• Subjects: Aspergillus
• ISSN: 0041-008X

Aflatoxin B sub(1) (AFB sub(1)) requires bioactivation to AFB sub(1)-8,9-epoxide for carcinogenicity, and glutathione S-transferase (GST)-catalyzed conjugation of activated AFB sub(1) with glutathione (GSH) is a critical determinant of susceptibility to the mycotoxin. Incubations containing [ super(3)H]AFB sub(1), rabbit liver microsomes, an NADPH-generating system, 1 mM GSH, and GST-containing lung or liver cytosol were performed to assess the abilities of lung and liver GSTs to conjugate AFB sub(1)-8,9-epoxide. [ super(3)H]AFB sub(1)-GSH was isolated by isocratic reverse-phase high-performance liquid chromatography (HPLC) and quantitated by liquid scintillation spectroscopy. Maximal [ super(3)H]-AFB sub(1)-GSH formation rates were significantly lower for lung than for liver (0.3 plus or minus 0.1 and 1.7 plus or minus 0.4 nmol/mg/hr, respectively). Immunoprecipitation of rabbit pulmonary cytosolic GSTs with antialpha or anti-mu GST antisera decreased [ super(3)H]AFB sub(1)-GSH production by approximately 45 and 51%, respectively, indicating that alpha-class and mu-class GSTs are of similar importance in catalyzing this reaction in the lung. Because mu-class GSTs comprise only a small proportion of total lung GST content, these enzymes have high specific activity toward AFB sub(1)-8,9-epoxide. In contrast, the pi-class GST appeared to play a negligible role. Using a rat liver microsomal system to generate both AFB sub(1) exo- and endo-epoxide isomers, and analysis based on chiral HPLC, we found that rabbit liver cytosolic GSTs catalyzed formation of both AFB sub(1) exo- and endo-epoxide-GSH conjugates, whereas pulmonary cytosolic GSTs catalyzed formation of only the exo stereoisomer at detectable levels. Despite a preference for conjugating the more mutagenic AFB sub(1) exo-epoxide isomer, the relatively low capacity for GST-catalyzed detoxification of bioactivated AFB sub(1) in lung may be an important factor in the susceptibility of the lung to AFB sub(1) toxicity.